1. Field of the Invention
The present invention relates to a method and apparatus in which active material elements are used in injection molding machine equipment (e.g., hot runner nozzle assemblies) in order to adjust a vent gap within a mold. “Active materials” are a family of shape altering materials such as piezoactuators, piezoceramics, electrostrictors, magnetostrictors, shape memory alloys, and the like. In the present invention, they are used to adjust the vent gap within an injection mold, thereby improving the quality of the molded article. The active material elements may also be used as sensors.
2. Related Art
Active materials are characterized as transducers that can convert one form of energy to another. For example, a piezo actuator (or motor) converts input electrical energy to mechanical energy causing a dimensional change in the element, whereas a piezo sensor (or generator) converts mechanical energy—a change in the dimensional shape of the element—into electrical energy. One example of a piezoceramic transducer is shown in U.S. Pat. No. 5,237,238 to Berghaus. One supplier of piezo actuators is Marco Systemanalyse und Entwicklung GmbH, Hans-Böckler-Str. 2, D-85221 Dachau, Germany, and their advertising literature and website illustrate such devices. Typically an application of 1,000 volt potential to a piezoceramic insert will cause it to “grow” approximately 0.0015″/inch (0.15%) in thickness. Another supplier, Midé Technology Corporation of Medford, Me., has a variety of active materials including magnetostrictors and shape memory alloys, and their advertising literature and website illustrate such devices, including material specifications and other published details.
FIGS. 1a-5 show a prior art mold to explain the venting problem. FIGS. 1a-1c show three views of a mold. The left view is the plan view of the core side, the right view is the plan view of the cavity side. The center view shows a section through the closed assembled mold. The mold comprises a cavity block 410 and a core block 411 and several ejector pins 412, 413 and 414. Both mold haves contain cooling channels 415 and 416. The cavity block 410 contains a vent 417, vent collector channel 418 and vent exhaust passage 419. The cavity block 410 also contains a melt sprue channel 420 for introducing the melt. The core block 411 contains a melt runner 421, gate 422 and sprue puller 423 machined in the core block 411. The closed mold encloses the mold cavity 424 which will form the part to be molded.
FIG. 2 shows the plastic material being injected into the closed mold cavity entering via sprue channel 420, runner 421 and through gate 422. As the resin 425 begins to fill the cavity 424, it displaces the air 426 that previously occupied that space. The melt pushes the air ahead of its flow path. Vent 417 has been positioned in the mold to provide a passageway for the air to escape and for this passageway to remain open until the resin has completely filled the mold cavity 424. Thus the vent 417 is usually positioned at a part of the mold cavity 424 periphery usually the furthest distance from the gate 422, the point at which the resin enters the mold cavity. If the vent were to be positioned at some other point the incoming resin may reach the vent, blocking it off and prevented any remaining air in the mold cavity from escaping through it.
The vent 417 is sized such that when the resin reaches that location it will not flow into the vent or the vent collector 418 beyond it. The vent gap 430 is typically 0.025 mm-0.075 mm (0.001″-0.003″), which is a large enough space to allow air to pass through, but a small enough space to prevent most resins from being able to flow therethrough. The depth of the vent is called the land 431 and is typically 0.625 mm-1.250 mm (0.025″-0.050″). The vent collector 418 is a much larger channel behind the vent 417 to allow unrestricted passage for the air that has passed through the vent. Vent exhaust passage 419 connects the vent collector 418 to the mold exterior so the air can exhaust to ambient conditions. The exhausted air exits the mold as indicated by arrow A in FIG. 2. When the injected melt reaches the vent it is too viscous to enter the small gap. FIG. 3 shows the filled cavity. FIG. 4 shows the mold opening and the ejector pins activated to push the solidified part 441 off the core half of the mold.
FIG. 5 shows what happens when the injected melt enters the vent and vent collector. This can happen if the melt injection pressure is high enough to overcome the clamping force holding the mold closed and the mold halves are forced apart, consequently increasing the vent gap and allowing the melt to enter. Alternatively, the viscosity of the melt being processed may happen to be much lower than that for which the vent gap has been designed. Sometimes this molded flash 440 remains attached to the molded part 441 and is ejected with it as illustrated in FIG. 5. On other occasions, the flash breaks off and remains in the vent and vent collector blocking them for the next molding cycle, and consequently the mold venting functions poorly and may result in a defective part being molded.
Another vent problem that may occur is when the vent gap is reduced or eliminated by hobbing of the mold. Because the vent is positioned on the mold's parting line 490 the repeated opening, closing, and clamping of the mold, as it cycles, can cause the parting line surface to gradually collapse. The effect of this is to reduce the vent gap. Periodically as molds wear their vents are remachined to restore the correct vent gap. When the vent gap is reduced or eliminated, the resulting poor or no venting of the mold cavity during the injection process may cause defective parts to be molded.
A Plastics Machinery & Equipment article by William J. Tobin, titled “Venting from the Inside”, contains a general overview of venting in injection molds.
U.S. Pat. No. 5,238,389 to Brandau et al. discloses a blow mold clamp mechanism for closing blow mold halves to an adjustable closed position, leaving a predetermined gap therebetween to act as a vent. In blow molding, the material in the cavity is a heated parison or preform that is being expanded in size by a compressed fluid in order to conform to the cavity shape. While there is a need to vent the mold to exhaust the air displaced by the expanding preform, the risk of material entering the vent gap is much lower than it is in an injection mold in which the material is in a heated fluid condition.
U.S. Pat. No. 4,489,771 to Takeshima et al. discloses means for automatically closing the vent of an injection mold when the injected material reaches the vent. A complicated, space-consuming mechanism is used at each vent location to preform this function.
EP 0 448 855 to Ryobi discloses a gas vent control valve for opening and closing a gas vent passage in a mold. The time taken from when the vent is signaled to close until it is actually closed is measured and compared to a preset period. If the actual time taken exceeds the preset period, an alarm is sounded, signaling an abnormality in operation.
U.S. Pat. No. 4,995,445 to Shigyo discloses a gas vent valve in a mold that is operated to close the vent passage in response to a pressure from the molten material in the mold cavity. A complex, space-consuming mechanism is mounted in the mold at the periphery of the mold cavity.
U.S. Pat. No. 5,397,230 to Brew discloses a vent apparatus for a mold comprising a reciprocating pin. The vent pin is responsive to a full resin level in the mold cavity to close the vent opening. While the vent pin is closed, cleansing fluid is circulated through the vent passage to clear resin debris prior to it hardening. The apparatus comprises a comparatively large pin and operating cylinder arrangement attached to the side of the mold cavity.
U.S. Pat. No. 5,683,730 to Katsumata et al. discloses a mechanically-operated closeable vent arrangement. There is a detection chamber that reacts to the incoming melt pressure and moves to operate a pin that closes the vent. A relatively large apparatus is used that takes up space in the mold structure.
Thus, what is needed is a new technology capable of closing a vent passage in a mold when the incoming melt material reaches the vent means, preferably including adjustable control, and preferably with embedded sensors and closed loop control of the closing function.